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A collaborative experimental study conducted at Lawrence Berkeley National Laboratory discovered a method to synthesize molecules known as polycyclic aromatic hydrocarbons, or PAHs.

PAHs exist both on Earth and in space and can be toxic byproducts of burning hydrocarbon fuel. According to Ralf Kaiser, professor of chemistry at the University of Hawaii at Manoa and a lead researcher in the study, PAHs can form in space as building blocks for carbonaceous nanoparticles, which could potentially contain biomolecules.

The results of the discovery help scientists understand how to eliminate the formation of toxic byproducts on Earth, according to Kaiser.

“We know now one pathway how (PAHs) can be formed, and hopefully, through this pathway, we can optimize combustion models and effectively design combustion engines to minimize the emission of this toxic product,” Kaiser said.

The study was conducted at Berkeley Lab’s Advanced Light Source research facility and funded by the U.S. Department of Energy.

The research team was led by Kaiser as well as Musahid Ahmed, a UC Berkeley scientist in Berkeley Lab’s Chemical Sciences Division. Additionally, Florida International University professor of chemistry and biochemistry Alexander Mebel carried out supportive electronic structure calculations.

“Our goal was to find out how can we form these aromatic structures in extreme high-temperature environment on Earth and also in space,” Kaiser said.

Through experimentation in a chemical microreactor, the researchers found that one possible mechanism for the synthesis of PAH was by the recombination of two radical molecules that have unpaired electrons, according to Kaiser.

“We conducted a reaction under controlled high-temperature conditions to really show, for the first time, that two radicals — in this case the indenyl radical and the methyl radical — can actually form the simple polycyclic aromatic compound, which is naphthalene,” Kaiser said.

According to Kaiser, over the last two decades there have been many postulated or theorized pathways for the synthesis of PAHs. These mechanisms, however, were based on calculations, incomplete models or paper chemistry — and there was no explicit evidence of the process occurring.

“Not many postulations became true, simply due to lack of sophisticated experiments,” Kaiser said. “We can provide solid evidence that one of the pathways, namely the recombination of two radicals, can form aromatic structures. … to put a really solid understanding on combustion model rather than speculation.”

According to Kaiser, the discovery of this mechanism can also help predict where these aromatic structures are formed and how carbonaceous nanostructures are formed in space.

There is ongoing research to continue discovering new PAH synthesis mechanisms. One of Kaiser’s postdoctoral students is currently conducting further reactions at the Advanced Light Source.

“In the future, more radical reactions will be studied,” Kaiser said. “It’s a long-term project to investigate the formation of aromatic structures by radical reactions.”